What are the Consequences of Leaving Lights on 24/7

kandm(8b coastal alabama)March 3, 2009

My seedlings are just shy of 3 weeks old. Almost all have their first true leaves and several have their second true leaves. I have a 2 bulb t12 set up and I leave the lights on all the time. I turn a fan on them a few hours a day and they seem to be doing well. I have heard you shouldn't leave the lights on all the time like I'm doing and I wonder why not?

Does anyone know what will happen to seedlings if they are under the lights full time?

From what i've read plants need a nighttime (ie: rest cycle)
Think of trying to be awake and active 24/7! LOL ...sure
it could be done for awhile but eventually your productivity
level is gonna drop off (same thing with a plant but not
nearly as extreme ofcourse)

Basically at some point with 24/7 lighting you will be just
wasting electricity...even if you just give a few hours
of 'night' it'd be better

If you're just using the lights for producing starts for the garden, it will be fine. In fact, it will produce larger, sturdier plants. If you're planning on growing the plants to maturity under the lights, that's a different story and I'll let someone who grows them under lights answer.

Continuous light caused some leaf deformities (wrinkles) but no chlorosis in sweet pepper grown in greenhouses. Although long term use of continuous light is detrimental to tomato and pepper plants, tomato and sweet pepper plants can take advantage of the extra light energy provided by continuous lighting for a short period of time. Early vegetative growth and fruit production of tomato and pepper plants were generally improved under continuous light compared the 14-h photoperiod (Demers et al., 1998a, 1998b). However, after that initial period, plants under continuous light grew more slowly than plants exposed to 14-h photoperiod; so that tomato and pepper plant growth and yield under 14-h photoperiod were then equal to or higher than under continuous light at the end of the experiment.

Costes et al. (1970) also observed that continuous light improved the early performance (hastening of flowering and fruit set, increased early yield) of sweet pepper plants compared to a 15-h photoperiod. Therefore, it might be possible to use continuous light for a short period of time (5 to 7 weeks) to improve growth of tomato and sweet pepper, especially during the months with the lowest natural light levels (December and January). However, such a practice should be investigated in order to determine if short term use of continuous light might have residual negative effects on tomato and sweet pepper plants.

NEGATIVE EFFECTS OF LONG PHOTOPERIODS AND THE FACTORS INVOLVED IN THEIR DEVELOPMENT
Tomato and sweet pepper plants do not take advantage (no increase in yield) when grown under photoperiods longer than 14 h (tomato) or 20 h (pepper). Tomato plants, but not sweet pepper, develop leaf chlorosis under continuous light. In the next sections, we will examine the role of the carbon metabolism, pigments, light spectral quality and day/night temperature differential in the development of these negative effects of long photoperiods.

Carbon Metabolism
High starch and soluble sugar accumulations were observed in leaves of tomato plants grown under long photoperiods, and it was suggested that these accumulations could be related to the development of the leaf chlorosis (Bradley et al., 1985; Logendra et al., 1990; Dorais, 1992).

Studies on other species support the hypothesis of a relationship between leaf chlorosis development and starch and sugar accumulations. For example, continuous light caused increased leaf starch and hexose accumulations and leaf chlorosis of eggplants (Solanum melongena L.) (Murage et al., 1996). However, eggplants growing under continuous light but in a CO2-free atmosphere for 12 h per day accumulated less starch and hexoses, and did not develop leaf chlorosis.

Exposure of tomato and sweet pepper plants to continuous light resulted in increased foliar contents in starch in tomato and sweet pepper, in hexoses (glucose and fructose) in tomato and sucrose in sweet pepper (Dorais et al., 1996; Demers et al., 1998a, 1998b). However, the reduction of the number of fruits on the plants did not modify the pattern of accumulation of starch and sugars in leaves of tomato and sweet pepper plants exposed to photoperiods of 14 and 24 h (Demers et al., 1998a, 1998b). Moreover, the reduction of the number of fruits on the plants did not influence the severity nor the date of appearance of the foliar chlorosis in tomato plants grown under continuous light. This indicates that accumulations of starch and soluble sugars are not caused by a limiting sink capacity. If there is a relationship between the excessive starch and soluble sugar accumulations and the development of the negative effects (leaf chlorosis, decreased growth and productivity) of the long photoperiods on tomato and sweet pepper, it is most likely a limitation of the carbon metabolism at the leaf level which is responsible for these accumulations.

In tomato, the use of continuous light caused, in addition to the foliar chlorosis and increased foliar contents in starch and hexoses, a reduction of the photosynthesis rate and of the activity of the sucrose phosphate synthase (SPS) enzyme (Demers, 1998). These reductions in photosynthesis and of SPS activity occurred between 6th and 8th week
under continuous light, i.e. about at the same time as the foliar chlorosis appeared, while starch and hexoses contents in leaves increased during the first 4 weeks of the experiment.

Since the reduction of the SPS activity occurred after the increase in starch and hexoses, it is thus impossible that the reduction of the SPS activity is responsible for these accumulations. However, it is possible that the SPS activity in vivo is limiting, which would explain the hexose increase. This suggests the limiting step of the export of photosynthates is the synthesis of sucrose in tomato and would explain the absence of growth and the productivity increase under continuous light. Furthermore, the increased hexose levels in the cytoplasm, by a feedback effect, would limit the export of the triosephosphate (photosynthesis products) out of the chloroplast, which would then be redirected towards starch synthesis, thus explaining the increased starch contents.

Moreover, the increased accumulation of starch would generate, by a feedback effect, an overload of the Calvin cycle, which would gradually cause the observed decrease of the CO2 fixation rate. Are the starch accumulations responsible for the leaf chlorosis in tomato? It is possible that the overload imposed on the Calvin cycle (decreased photosynthesis) could limit the use of the reducing potential (ATP, NADPH) produced by the luminous phase of photosynthesis, thus causing an overload on the electron transport chain and the photo-oxidation of the chlorophylls (decrease in the leaf chlorophyll contents), and thus explaining the observed leaf foliar chlorosis. Transgenic tomato plants (in which a gene coding for the SPS enzyme was incorporated and overexpress this enzyme) could be used in future studies to test if accumulations of starch in leaves are responsible for the development of chlorosis observed in tomato plants exposed to continuous light. Transgenic tomato plants (overexpressing SPS) have higher photosynthesis rates and accumulate less starch and more sucrose than non-transformed
plants, especially under conditions of saturating light and CO2 (Galtier et al., 1993, 1995; Micallef et al., 1995). One can put forth the assumption that, under continuous light, leaf starch contents would be lower in transgenic plants than in normal plants. If this is the case, the reduction of the leaf starch content in transgenic plants should thus prevent the development of the leaf chlorosis, or at least decrease its severity.

In sweet pepper, the use of continuous light caused an increase in the leaf starch and sucrose contents, but did not affect leaf hexose contents, photosynthesis rates and SPS activity (Demers, 1998). The increased foliar contents in sucrose indicate that SPS activity in sweet pepper is not limiting as in tomato. Increased accumulation of starch in
sweet pepper plants exposed to continuous light would be explained by the fact that continuous light results in a longer period of time over which starch synthesis occur, but without overloading the starch synthesis pathway. Thus, starch accumulation in sweet pepper under continuous light would not be important enough to cause a reduction in CO2 fixation (no overload of the Calvin cycle). Increased leaf contents in sucrose suggest that sucrose export would be possibly limiting. In sweet pepper plants, the export rate of carbon (as sucrose) out of the leaf is constant, and the export rate would be limited at the level of the loading of sucrose in the phloem (Grange, 1985, 1987). This would explain why the growth and the productivity of the sweet pepper plants do not increase under continuous light.

Pigments
In growth chambers, continuous light caused leaf chlorosis, decreased photosynthesis rates, and reductions in leaf contents in pigments (chlorophyll a and b,
carotene, xanthophylls) in both tomato and sweet pepper plants (Demers, 1998). Leaf chlorosis, decreased photosynthesis rates and loss of pigments were more important and occurred earlier in tomato plants than in sweet pepper. Compared to sweet pepper plants, EPS ratio (epoxidation state of the pigments of the xanthophyll cycle) was lower in tomato, indicating a greater need for energy dissipation and a more important state of stress (caused by excessive light). Pigments such as carotene and xanthophylls (violaxanthin, antheraxanthin, zeaxanthin) play a significant role in the protection of the photosynthetic apparatus against damage that could be caused by an excess of light.

Carotene and xanthophyll levels were higher in sweet pepper plants than in tomato. Thus, sweet pepper has a better protection against the degradation of chlorophylls, which would explain why leaf chlorosis appeared later and were less severe in sweet pepper.

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Leaving lights on 24/7 is OK and perhaps beneficial (not counting the cost of electricity) for no more than seven and maybe as few as five weeks and after that it is detrimental.

Wordwiz,
I had a discussion with him a while back on the hot pepper forum. We agreed that the research focused on growing under lights, not starting under lights and that 24 hours for starting is benefitial with typical equipment.
Rick

Yeah, I know. I posted the original link and AJ downloaded the entire report!

For 5-7 weeks, continuous lighting appears to help production. My hope is to try an experiment this spring using an early producer plant. Four rooms, light running 13, 17, 21 and 24 hours per day, two plants per room, all growing hydroponically. I'm curious since I want to try to raise greenhouse toms this fall, winter and spring.

Mike,
I'm trying the same experiment right now. I have an extra shelf this year because I added it for overflow. Each shelf has 10 40w T12 bulbs and holds 4 trays. I put an extra tray of lettuce, broccoli, and peppers on a shelf that I only keep on 16 hours per day. The other shelfs I leave on for 24 hours. The peppers are the only real question regarding the study. The others will be under the lights much less than 10 weeks (I start my peppers for 10 weeks indoors). They were all started on Feb 1st. So far, here are my results:

1. Lettuce doesn't care. Both trays look the same.
2. The broccoli that is under 24/7 lighting looks much better and is ready for the garden in 4 weeks.
3. The peppers under 24/7 are much larger and stronger looking. The 16 hour peppers are about half the size as the 24/7 ones (3 sets of true leaves as opposed to 1). I'll see how they end up but so far 24/7 looks like the real winner.

I didn't follow your approach but it makes sense for just starting. I can get some pictures if you're interested.
Rick

The 24/7 tray is fuller and stockier. The 16/7 tray actually has the tallest plant but it's not as stout as the ones in the 24/7 tray. I do plan to go the full 10 weeks this way and see if I can tell a difference in the yield this summer.
Thanks,
Rick

Just to confuse the issue a bit, I have a tomato plant in a window at the office. It gets direct light for maybe six hours a day and ambient light for another six hours or so. In just over four weeks it has grown from four inches to over 20" and now has six or seven blooms on it.

Mike,
The difference is getting more evident every day. The one on the left is on the shelf that gets 16 hours of light per day. The one on the right is on the shelf that gets 24 hours of light per day. If I didn't want to see how it will affect yield I'd turn them all on 24 hours now.
Rick

Looks impressive. But please, don't stop the experiment, at least not yet. It looks like the five weeks have passed and getting closer to seven (if they sprouted on Feb. 1). But it will be nice to see how things look after 10 weeks.

>> Wordwiz, is that getting moved outside? I'd do it quick before those blossoms get too big. You'll miss the initial growth phase that happens after transplant. Nah, it's going to live out its life in that bucket of water on that shelf.

I suspect you will need to switch it to a 12/12 cycle after vegative stage/once maturity is reached. this is recommended for many plants grown under lights to start the fruiting stage.
failure to do so could produce little to no yield and i feel this will happen in tomatoes from memory.

I myself am also growing tomatoes under lights. It's going great for the new plant i just started. but unlike yours i will try the 12/12 stage when it is mature and will post my results so we can compare so more data aswell.

Elmondo,
I have never intended to stay on 24 hours of light forever on these plants. These are starts that will go out to the garden. The question (I am/we are) trying to answer is whether 24/7 produces better starts than 16 hours of light. My contention is that 24 hours is better for starts using fluoresent lights especially considering that the amount of light is very small compared to the sun. Others have argued that they need a rest period each night, especially after week 7.
Thanks,
Rick

Mike,
Yes, it's been a little over 8 weeks (since Feb 1). I'm a little surprised as well. I noticed a large slow down in the 16 hour seedlings and didn't see the same in the 24 hour ones in the last couple weeks. Actually, the 24 hour seedlings have gotten so large that I expect to see a slowdown because they are shading each other too much now. Out of the two trays I'd say the 24 hour are on average about 2" taller than the 16 hour ones. Hopefully, they will be going into the garden in 2-3 weeks, depending on the weather.
Rick

Mike,
I'll move a few of the 24/7 seedlings to the 16 hour shelf. Then I'll space out some of the 24/7 seedlings. Unfortunately, I will only be able to do it for a few since I don't have much room on either shelf. Thanks for the suggestion.
Rick

Rick,
Very interesting experiment. One question; are the 16hr peppers in complete darkness for 8 hours or do they see some weak light from the other lamps? My technique has been 24/0 from seed, and sometimes 18/6 for a while before they go outside. My main motivation for the 6 hour dark period is to save some power and keep temps below 85 in the heat of the day.
-Zander

Well, I just wanted to provide an update on the 24 hour experiment. All of the peppers are starting the hardening off process now so the indoor part of the experiment is over. They are now in a fairly sheltered place outside and will only be moved inside if temperatures drop below 38 degrees at night. I found a great way to harden them off last year. Since I have a large trailer that I use to move my tractor when needed I just put them all on the trailer and move the trailer to a place with more and more light until they are fully hardened off. Ok..the results.

The 24 hour peppers definitely look better. They look like they are a few weeks ahead of the 16 hour peppers. I'd say on average they are about 2" taller and have thicker stems. Some already are flowering and are probably past overdue to get in the garden. I'll mark them this summer and see if I can tell any difference in the yield or how fast they produce fruit. As expected there are some in the middle of the flats that were not getting enough light because they were pretty densely packed in 3" pots.

Overall, I'd have to say that 24 hour light is great for seedlings.
Rick

Rick~
Can I ask at what height you had your lights? This is the first year that I have put it chains to adjust my lights. I can try to post a picture of my set up later. I have always used 24 hour light (due to laziness to get a timer) but this year with lowering the lights I have had better growth. Thing is, I started them in a "DOME" with plugs, quickly they out grew the dome and plugs, I moved to three inch pots, but left them in the dome for moisture sake. Thing is the "DOME" is large and the lights above needed to be the height of the dome, so now my Tomato's are looking a bit leggy. should I uncap the dome and lower my lights?
Again I will try to post a picture a bit later...
I appreciate any one's help and feedback.
L

L,
I keep the lights 1 to 2 inches above the plants. Often the lights are touching the plants because they grow so fast. I do it by adjusting chains as well. I use a clear plastic cover until I get the first sprout in the tray and then take it off. I'm assuming thats what you mean by dome. Sounds like you should lower the lights ASAP.
Rick

Thanks.. Yes a certain seed company sells a Dome... that is what I use.
I have used the dome after sprouting becuase it helps with the moisture. I also use heat to start things off.
I probably started to late, but here in Michigan you never know when you can plant :) it snowed here last week, after 5 days of sun...
I will lower the lights today. I was thinking that they should not touch because they may burn but I will lower.
Laura

Very interesting, I recently read something about this somewhere else, and I heard from someone that Advanced Nutrients did a study that showed 24/7 lighting out-performed other schedules in plant growth.

But I think it makes a lot of difference what kind of plant you're talking about. If a plant is triggered to produce fruit based upon light cycle, too much light wouldn't necessarily be a good thing.

Plus you have to think about the electric bill. If you're not running very many or very big lights it might not make much difference, but if you've turned your home into a greenhouse it could cost a lot to run the lights all the time.

Sorry...bad explanation on my part. I'm starting cuttings in an outdoor shed where heat build-up is a concern during the day. I started cycling the lights on and off in 30 minute intervals during their 16 hour day...which effectively uses 1/2 the ammount of electricty while helping keep the propagation area cooler. There have been no adverse effects in shoot growth that I can see.

I'm suggesting there may be cost saving ways to achieve a 24hour light cycle that are worth a try.

24/7 Henry Kuska posted about this before. He floods his seedlings 24/7. I flood my rose cuttings 24/7. They grow nice and big, also if you are using fluorescent, the lamps last longer and have stronger output (even though this might be only marginal, it is still true) if they are on 24.7. Plus you maximize the output for your facility. Some people claim the plant needs to "rest", but this is not true, it only needs a dark period if it is photoperiodic, and for ourselves, we are
talking about VEGETATIVE phase, I assume. A PHOTOPERIODIC plant REQUIRES dark, in order for the flowering hormones (Pyr Pyfr) to mature and trigger flowering.
What Henry pointed out is that CYCLING OF TEMPERATURE can be important, when the lights are on 24/7, some plants do better if there is a cooling down time.

Just a quick update. The pepper plants that were given 24/7 light are already starting to produce! They are definitely larger and healthier looking. That's way ahead of last year and even ahead of my tomatoes. The 16 hour plants are definitely behind.

Mike,
I can definitely tell the difference. The 24/7 plants have almost all produced picking sized peppers already. I have a drawer full in the fridge. They are also putting on a second set of peppers that are about the same size as the 16 hour peppers. The only picking sized peppers from the 16 hour group are jalapenos and those are 42 picked from the 24/7 plants and 3 from the 16 hour plants. It looks like I'm going to get an extra crop from the 24/7 peppers. I think I have enough information to call the 24/7 peppers a big sucess.
Rick

New to the conversation...
Rick this experiment was interesting. You mentioned that the pepper plant was even ahead of your tomatoes...how much light did you give your tomatoes? Did you start them at the same time? I like to grow both peppers and tomatoes and I sure would like to be picking some in June next year. I am still waiting with only blooms so far.

Shawn Ann,
Your garden looks great! In upper zone 6 you're way ahead. I'm getting a couple of pounds of beans a day and a lot of peppers but that's about it for now. The cucumbers will be ready to start to pick in a couple of days and the tomatoes are just started. I picked the first Sungold today. We should have our first large tomato in about a week. We're also getting summer squash and zucchini. I don't think we have any watermellons forming yet. I'll have to check. The tomato plants were under 16 hour light this year. Next year they will be under 24 hour light but started a little later.

I'm glad you schooled everyone in the benefits of 24/7. I personally run 24/7 on my roses, and the growth is absolutely phenomenal. The only reason to run a night cycle, is for (in my experience) to grow cuttings. When cuttings have no roots, they cannot make use of 24/7 illumination. using 16/8 cycle allows for a cooling period and a dark period which may be of benefit to INITIATE roots (not grow roots - roots grow all the time). Also, for cannabis, in vegetative phase, 24/7 is also the way to go for a much bigger and heavier harvest, and nicer smoke, with very nice strong high.

Why why why. The picture indicates the plant with 24hr lighting is visibly larger, but a picture alone at this stage of growth is a somewhat poor indication of actual plant health. Just turn your lights off at night like everyone else and save electricity. FYI whoever thinks Alaska plants benefit from extended periods of daylight should think again, the extended light periods experienced during the summer season do not supply as many lumens as you might think.

The plants did grow quicker, I followed his thread throughout last spring. And while there may not be an advantage to 24/7 if one has plenty of time to get the seedlings large enough, it is a huge benefit if time is short.

I would like to see any updates. Interesting reading and what I've read and been told by several growers who have researched and experimented with 24 hour lighting. I had some I got started this year 7-10 days late. They have been under 24 hours lights for 11 days and have caught and starting to pass those 7-10 days older. I have them all under 24 hours lights now. From what I'm seeing the light bill won't be a lot difference because they are growing fast enough I will be able to move them to the cold frames and start hardening off sooner. If you had a greenhouse and wanted some growth before moving there it would work well also. I feel I will move them outside and turn the light shelves off 7-10 days earlier than normal. So the money I save there will offset some of the cost of keeping them on 24 hours straight. Again interested in all updates. Jay

The problem with how you tested 16/8 and 24/7 is that you are not using enough light.

At proper light levels & 24/7 lighting things will go badly. Sugars/energy will build up in the tops of the plant in a solid form and will not be mobile or usable to the plant. the amount of Fruit set will be lower and the quality will be much worse.

At the lower light levels it just so happends the 24/7 plants were just less abused than the 16/8 plants.

& depending on how well you set up the experiment could have made a big difference.... Did you light seal the room? ANY light over ~ 5lumens? that gets to the plants (at night time) the phytochrome a/b (what a plant uses to judge how much day or night time over the day it has had is instantly converted and the cycle must start again.

What's the current wisdom, is it ok to leave the lights on 24/7 for seedlings, or give them an irregular lighting cycle? I think the day/night cycle is more of an issue for full size flowering plants, or seedlings getting full sun, and it is perfectly fine for indoor seed starting to leave the grow lights on 24/7, correct?

The above picture shows the initial domed seed trays in November 2014. I commented elsewhere that I am growing hosta seedlings under fluorescent lights since middle of November 24 hrs each day. Some of my seedlings are shown in the Hosta Forum in the Seedlings thread. I will bring them outside in May. Common 4ft fuorescent lights were 1 inch above domes and are now 1 inch above tallest leaves of plants. All light gets reflected back to plants with aluminum foil covered cardboard, not shown yet above. I am doing this now the 4th year and have seen no problems. Another benefit is that the heat generated by the lights is enough to germinate under domes at 81 dgrs and grow the plants thereafter without domes at 71 dgrs in my 60 dgrs basement. I created enclosed growing chambers on my 2 shelves by enclosing above shelves with 2 walls (behind and right) and old towels (left and front). I no longer use my heating mat.